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Keratoconus
Introduction:
Is a bilateral,
noninflammatory corneal
ectasia with an incidence of
approximately one per 2,000
in the general population,
although less obvious cases
such as early forme fruste
presentations are thought to
be much more common.
Keratoconus is characterized
by a progressive increase in
corneal curvature, with
apical thinning and
irregular corneal
astigmatism. Eventually, an
obvious cone-shaped
protrusion of the corneal
surface may develop.
Keratoconus often becomes
apparent during the teenage
years and classically
progresses until the third
and fourth decades of life,
when many affected
individuals experience an
arrest of disease
progression or at least a
reduction in the rate of
progression.
Etiology
The pathophysiology of how
keratoconus develops and
progresses is not well
understood. Genetic factors
appear to be multifactorial
and are considered
fundamental to the etiology
and progression of
keratoconus. However, the
underlying molecular and/or
genetic abnormalities are
unknown.
Keratoconus has been linked
with systemic conditions
such as atopic disease,
genetic conditions such as
trisomy 21 and Turner’s
syndrome, and various
connective tissue disorders,
as well as with eye rubbing,
rigid contact lens wear and
ocular trauma. Keratoconic
corneas also have an
accumulation of cytotoxic
by-products from the nitric
oxide and lipid peroxidation
pathways, abnormal
antioxidant enzymes, and
increased levels of
mitochondrial DNA damage.
These findings suggest that
ongoing oxidative stress
contribute to keratoconus.1
Signs and Symptoms
Early stage. Very
early in the disease
process, the visual
impairment caused by
keratoconus is usually
correctable with soft
contact lenses or
spectacles. These patients
may gain the attention of
the ophthalmologist if
progressive myopic
astigmatism develops,
particularly with
steeper-than-average
keratometry or topography
readings. As the disease
progresses, it is often
increasingly difficult to
refract the patient to a
crisp and clear visual
acuity with soft contact
lenses or spectacles.
Intermediate stage.
As the disease progresses to
an intermediate stage,
patients usually experience
vision loss that is no
longer correctable with soft
contact lenses or
spectacles. Toric soft
lenses often help with
correcting some of the
astigmatism; however, the
increasing irregularity of
the astigmatism may call for
rigid, gas-permeable
contacts in order to achieve
truly crisp vision. Some
patients require a piggyback
configuration consisting of
hard contact lenses worn
over soft lenses to achieve
adequate fit, comfort and
vision. If a young patient
develops progressive
astigmatism between
examinations, this should
alert the ophthalmologist
that further corneal imaging
may be needed to evaluate
the patient for worsening
keratoconus.
Advanced stage. For
patients who progress to the
more advanced stages of
disease, contact lens wear
may become increasingly
difficult and often
uncomfortable due to the
steepness of the cornea and
difficulty in fitting the
lenses. Contact lens
intolerance is a common
indication for corneal
transplantation at this
stage.
Diagnosis
Early stage.
Computerized corneal
topography (CCT) techniques
using curvature-based
analysis and newer forms of
elevation-based tomography
appear to be the most
sensitive methods for
detecting early keratoconus.
Prior to these, the most
sensitive methods included
1) apical decentration on
the keratometer using the
Soper topogometer, 2) subtle
changes in the retinoscopic
reflex that progress to
early scissoring and 3)
changes in the ability to
visualize the endothelial
specular reflection relative
to the epithelial reflection
during slit-lamp
biomicroscopy.
Elements of earlier
techniques- keratometry and
Placido disk–based
keratoscopy-have now been
integrated into
sophisticated, computerized
systems capable of
generating color-coded
curvature and elevation
maps. Even with these
technological advances, it
is increasingly recognized
that elevation data using
scanning slit or Scheimpflug
imaging provides data that
are diagnostically very
helpful when screening
patients with possible
corneal ectasia.
A number of diagnostic
algorithms can help diagnose
early keratoconus; however,
there remains no universally
agreed-upon diagnostic
criterion that, by itself,
conclusively diagnoses the
earliest cases. Typically,
changes in elevation
relative to a best-fit
sphere occur on the
posterior corneal surface
more rapidly than similar
changes are seen on the
anterior surface. Other
findings include apical
decentration, corneal
thinning, increasing
irregular corneal
astigmatism, and Rizzutti’s
sign, which is a sharply
focused conical reflection
obtained on the nasal cornea
after a penlight is shone on
the temporal side.
Intermediate stage.
Even in patients with
intermediate-stage disease,
corneal changes may still be
subtle and not readily
apparent by slit-lamp
biomicroscopy alone. The
retinoscopic streak usually
demonstrates scissoring, and
it is hard to neutralize the
streak reflection during
refraction. Computerized
corneal topography and
elevation-based
tomography-Orbscan IIz
(Bausch & Lomb) and Pentacam
(Oculus) are probably the
most widely used-typically
demonstrate findings that
are usually more obvious
with regard to both
diagnosis and monitoring
disease progression.
Advanced stage. In
more advanced cases,
keratoconus is readily
diagnosed by characteristic
slit-lamp findings:
-
Stromal
thinning-usually
inferiorly.
-
Apical
decentration and conical
protrusion.
-
A
Fleischer ring-an
iron line within the
deep epithelium
surrounding the cone at
the base.
-
Vogt
lines-fine vertical
lines in the deep stroma
and Descemet’s membrane
that disappear
transiently with gentle
digital pressure.
In addition to these
corneal signs, external
ocular manifestations
include:
-
Munson’s
sign. The lower lid
protrudes when a patient
with advanced corneal
disease looks downward.
-
Corneal
hydrops.
Occasionally, patients
can progress to acute
corneal hydrops, a
stromal edema caused by
aqueous penetration
through breaks in
Descemet’s membrane. The
onset of corneal hydrops
is classically
associated with a sudden
decrease in both
uncorrected and
corrected vision as well
as redness, pain and
photophobia. The corneal
edema may persist
unpredictably for weeks
or months, with gradual
resolution as the
posterior break is
replaced with a
posterior collagenous
scar. Topical osmotic
agents such as
hypertonic saline may
accelerate the recovery
of vision; however, this
treatment is most
helpful to alleviate
edema within the
epithelium and anterior
stroma.
Treatment
There are no drugs known to
reverse or prevent
keratoconus, but patients
may be able to slow its
progression by refraining
from rubbing their eyes.
When spectacles or contacts
are no longer effective,
there are several surgical
options, including the
following:
Penetrating keratoplasty.
Full-thickness penetrating
keratoplasty (PK) is the
treatment most commonly
offered. Nearly 95 percent
of these procedures maintain
a clear and functioning
graft at five years. The
rejection rate appears to be
lower than that seen in
patients undergoing PK for
other indications. In spite
of successful surgery,
residual corneal astigmatism
and refractive error often
limits the corrective power
of spectacles and usually
requires additional
correction with a contact
lens. In addition to
irregular astigmatism,
complications after PK can
include allograft rejection,
a fixed, dilated pupil and,
on occasion, recurrence of
keratoconus. What causes
recurrence is unclear; it
may represent underlying
disease in the donor or
possibly progression of
disease.
Lamellar keratoplasty.
For patients who have
moderate keratoconus without
significant scarring, there
is renewed interest in
lamellar keratoplasty,
especially with the
precision, predictability
and convenience of the
femtosecond laser for these
cases.
Corneal ring segments.
Corneal stromal ring
segments (Intacs, Addition
Technology) are also an
option, particularly if the
patient demonstrates disease
progression with apical
displacement. These ring
segments consist of two
150-degree PMMA arcs that
are inserted surgically into
the cornea through a small
1.8- to 2-mm radial incision
created in the steep axis.
Adding ring segment
thickness to the
midperipheral corneal stroma
produces an “arc
shortening,” effectively
flattening the central
cornea. This results from
the corneal lamellae
extending from limbus to
limbus in a continuous
fashion with a new
alteration in corneal
curvature that is in
proportion to the ring
thickness. The thin,
ectatic, keratoconic cornea
is particularly amenable to
this effect.
Complications have primarily
been associated with the
insertion technique, but
technique has improved
greatly due to the use of
new equipment designed to
reduce the risk of poor ring
placement. Although the ring
segments lack great
predictability, most
properly selected patients
experience some improvement
in their uncorrected vision
and may regain contact lens
tolerance due to improved
corneal topography.
Recent studies have shown
that Intacs corneal ring
segment implantation is a
safe and effective treatment
for keratoconus, with
significant and sustained
improvements in vision as
well as improved contact
lens tolerance.2 Studies
have also compared placement
of single-ring with
double-ring segments. In
some cases, the single-ring
segment offers improved
astigmatic correction, but
the evidence is not
conclusive.3 Studies are
also under way examining the
combination of ring segments
with collagen cross-linking
using riboflavin activated
by ultraviolet irradiation.
Contraindicated surgeries.
LASIK remains
contraindicated in patients
with keratoconus due to the
possible risk of
accelerating the ectasia.
Surface ablation
photorefractive keratectomy
procedures are also
considered contraindicated
in patients with known
keratoconus; however, this
surgery is currently under
study in patients with early
disease that is considered
stable or slowly
progressive.
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